Image tube



May 2, 1950 L. E. FLORY ETAL IMAGE TUBE Filed Oct. 5, 1946 y VIII] Snventors LIZ-EUL-EF'LDRY Eeurslz-AMURTDN 5 L nHN E-.IRuEDY HWLM Gttorneg Patented May 2, 1950 IMAGE TUBE Leslie E. FloryV and John E. Ruedy, Princeton, N. J., and George A. Morton, Knoxville, Tenn., assignors to Radio Corporation of America, a

corporation of Delaware Application october 5, 194e, serial-ivo. 701,588

5 Claims. (Cl. Z50-1645) Our invention relates to an electrooptical image or viewing tube and more particularly to image tubes in which an electron image from a photocathode is electrostatically focused upon a fluorescent screen.

Image tubes of the electrostatically focused type have been made which have between a photocathode and a fluorescent screen a series of spaced annular anodes for focusing an electron image from the photocathode on a fluorescent screen. These tubes as heretofore made are rather bulky and not well adapted for use in portable equipment or for commercial production.

An object of the invention is to provide an improved image tube in which the image on the screen is reduced in size and is magnied by a lens to give a light image of greater brightness to the eye of the observer than would be the case if the image on the iiuorescent screen were of the same size as the magnied image.

Another object of the invention is to provide an image tube in which the pin cushion effects are reduced by the shape of the optical lens through which the image is viewed.

Another obiect is to provide an image tube which is compact, mechanically strong, and suitable for manufacture on a commercial scale.

The novel features which we consider characteristics of our invention are set forth with particularity inthe appended claims but for a better understanding of our invention itself both as to its organization and method of operation together with other and further objects and advantages reference may be had to the following description of a specific embodiment, shown p,

merely for illustration, taken in connection with the accompanying drawing in which:

Fig. l is a diagrammatic longitudinal section oi. one form of embodiment of our invention;

Fig. 2 is e, side view partly in longitudinal section showing various details of construction; and

Fig. 3 illustrates pin cushion effects and the correction thereof.

In accordance with our invention we provide an Aimage tube of the electrostatically focused type, which forms on the target or fluorescent screen an image smaller than the optical image projected on the photocathode and in which the light image on the screen is viewed by the observer through a magnifying hemispherical lens mounted with its fiat side substantially coincirient with the plane of the screen. In the preferred construction we provide between the photocathode and the target a plurality of annular cylindrical anodes or focusing electrodes which increase in length and preferably decrease-'-v in diameter progressively from the cathode to thel target and are maintained at positive potentials which likewise increase progressively in the same direction.

tensied electron image on the screen and thus produces a light image which is brighter and more intense the smaller the electron image. If

this light image is magniiied by viewing through a conventional eye piece type magnifier, there isv no lossin brightness since the eye is the limiting aperture of thesystem and, consequently, to the observer there is a net gain in brightness inversely"` proportional to the ratio of areas of the originalY optical image on the photocathode and the re' duced light image-on the screen. The reduction in size of the light image and consequent increase in brightness is limited only by mechanical limitations in structure of the electron lens and by the degreeof magnification which can convencauses the sides of the image of a square to apf-A pear on the screen to be convex so that the image,A resembles a pin cushion. It has been impossible to entirely eliminate this effect by curving thephotocathode without introducing equally objectionable photooptical defects. compromise has usuallyl been made by curving the photocathode to partially eliminate pin cushion effect. In our invention We substantially complete the elimination of pin cushion effect by using a particular form of optical lens for magnifying the light image produced on the fluorescent screen by the photoelectrons. The lens constructed for this purpose is a hemisphere in form and it is mounted in the tube, with its at side approximately coincident with thel screen. The hemispherical lens tends to cause the sides of a square image to appear somewhat concave toward the center of the square, and thus corrects for the pin cushion distortion and may have, for example, a magnification of about 1.5. As a result a lower power eye piece type magnifier with a consequent greater aperture may be used in the eye piece through which the light image is observed.

Referring the drawing,` the tube envelope" I is preferably made in two sections of different diameters which are joined at their proximate These annular anodes or focusing electrodes constitute in effect an electrostaticelectron lens which produces a reduced and in- Therefore, a

a rigid structure by a plurality of insulating beads or spacers 9, I and I I. The transparent photocathode I2, Which may be of the caesiated' silver oxide type, for examplais mounted at, and preferably formed on the transparent end Wall of the larger section of the envelope remotefrom the annular flange, and the vfluorescent screen or target I3 is placed atl the end of the smaller section remote from the annular flange. The anode or focusing electrode 5, which is at the highest positive voltage, is preferably Within the smaller section of the envelope'and extends beyond the annular flange into the larger section.

This construction facilitates the assembling of the anodes or focusing electrodesv into a unitary mount on a stem consisting of the Vsmaller section of the envelope With`V the annular flange E, the edge of the annular flange thenl being sealed to the walls of the larger section of the envelope. 'By means of the leads in the annular vflange 2 the photocathode and' the focusing e1ectrodes may be electrically connected to a source ofv voltage, such as a battery, as diagrammatically shown inFig. 2.

lllhe Way' oui` invention overcomes pin cushion effect may best be. understood by reference to Fig. 3. The optical image of va square on the photocathode I2 is distortedV so that it appears on the fluorescentrscreen to have concave sides A. A squaregimage` :receivedv through the hemiyspherical lens is distorted S0 that itappears` to have convex sides B. These distortions are 0pposite and substantially equal in amount, so that the convex sides .A appear to be straight, andthe pin cushion effect is corrected so that the image appears square, as at C. Eye pieces or other lenses (not shown) are used to further magnify the image tothe desired amount.

To produce a visible light image the Vscreen may be of willemte or other phosphor which will fluoresce in response to electron bombardment.

The photocathode maybe made responsive to V observed may be of glass for visible light or near infra-red, o quartz for ultra-violet light, or oi some material such as hard rubber or rock salt for infra-red light.

Inaccordance with our invention the annular anodes Vor focusing electrodes'coaxially aligned between the photocathode and the target progressively decrease in diameter and increase in length with their distance from the photocathode toward the target, and constitute an electrostatic condensing lens for the electron image produced by anY optical image on the photocathode.

For best results the visible Vimage on the iiuorescent screen should' be of about the same rliameter as the hemispherical lens I4 through which the visible image on the screen i3 is viewed by the observer.' The hemispherical lens may for convenience be made of glass and be integral with and constitute the end of the smaller section 2 of the envelope. The flat side of the lens I4 faces the photocathode, and the screen I3 may be deposited on and be coextensive with the fiat side of the lens I4. In one type of successful tube the photocathode I2 is about one and a half inches in diameter and the hemispherical lens I4 about one half inch in diameter. The electrostatic condensing lens focuses the electron image on the screen and reduces the focused image to about the diameter ofthe lens I4.

The invention is not limited to a tube of any particular dimensions but by way of example it may be stated that -We have found it satis- `factory to construct the tube with the following dimensions.:

, Inches Radius of curvature photocathode 2.38 Diameter of anode cylinder 8 1.30 Length of anode cylinder l! .25 Diameter of anode cylinder l 1.208 Length of anode cylinder 'i .500 Diameter of anode cylinder 5 1.163 Length of anode cylinder t .375 Diameter of opening l5 .725 Diameter of anode cylinder 5 .642 Length of anode cylinder 5 1.750 Gap between anodes 5 and t .125

The dimensions ci the focusing anodes. may be varied over relatively Wide ranges, althoughfin accordance with our invention these anodes should decrease in diameter and increase in length progressively from the photocathode to the target, and the approximate relationshipv of length to diameter above explained should be. maintained for the best results.

As a number of possible embodiments maybe made of our invention and changes may be made inthe embodiment described, it Vis to be understood that Whilesorne illustrative forms of our invention havebeen described in considerable detail, our invention is not limited to such details, since many changes and modifications may Well be made Without departing 'from the spirit or scope of the invention in its broadest aspect'.

What we claim as new is:

l. An image tube having acylindri'cal envelope comprising two tubular coaxial sections of. different diameters joined by a flange at their proximate ends, a photocathode at the other end of the larger section, a fluorescent screen at the other end of the smaller section, an electrostatic focusing lens between said photocathode and said screen comprising a plurality of tubular lens electrodes diilerent progressively in increased Y length and decreased diameter with distancefrom said photocathode, the electrode of least length and greatestv diameter being adjacent said photocathode and the lens electrode of greatest. length and smallest diameter positioned `in and concentric with the smaller section of said envelope with one end adjacent said screen and the other end projecting beyond the end of said smaller section and into the larger section of said envelope, and leads sealed into and extending through said flange and supporting said lens electrodes in said envelope.

2. An image tube comprisingv a cylindrical evacuated envelope having a transparent Window' at each end, a translucent photocathode adjacent one of said windows and adapted to produce in said envelope an electron image of an optical, image on said Window, a planar luores'cent'screen adjoining said other Window, a plurality of co4 axial cylindrical tubularelectrodes spacedv along said envelope in a row between said photocathode and said screen and constituting an electrostatic lens for focusing said electron image on said screen, an electrode of least length and greatest diameter being adjacent said photocathode and the length of each succeeding electrode being greater and its diameter less with increased distance from said photocathode.

3. An image tube comprising a curved photoelectric cathode for producing an electron image corresponding to an optical image on said photocathode, a plane fluorescent screen facing said photocathode, an electrostatic condensing lens between said photocathode and said screen for focusing said electron. image on said screen with some pin cushion distortion to produce on said screen a correspondingly distorted visible image smaller than the optical image on said photocathode, and a hemispherical lens of substantially the diameter of said visible image adjoining said screen with its ilat side substantially coplanar with said screen and producing an optical distortion substantially equal and opposite to the pin cushion distortion of the visible image on the screen.

4. An image tube comprising an envelope, a curved photoelectric cathode at one end of said envelope for producing an electron image correspending to an optical image thereon, a glass hemispherical lens smaller than said photocathode sealed to the other end of said envelope with its flat side facing said photocathode, a iluorescent screen on and coextensive with the fiat side of said glass lens, and an electrostatic condensing lens between said photocathode and said screen for focusing said electron image reduced to the size of said screen and coextensive therewith.

5. An imagel tube having a cylindrical envelope comprising two tubular coaxial sections of ldiierent diameters joined at their proximate ends, a. photosensitive cathode adjacent the other end oi said larger section, the other end of said smaller section consisting of a glass hemispherical lens with its flat side facing said photosensitive cathode, a fluorescent screen on and coextensive with the at side of said glass lens, and an electrostatic condensing lens comprising a plurality of spaced and coaxially aligned annular electrodes between said screen and said photosensitive cathode for producing on said screen a visible image coextensive with said screen.

LESLIE E. FLORY.

JOHN E. RUEDY.

GEORGE A. MORTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,120,916 Bitner June 14, 1938 2,156,813 Kautz May 2, 1939 2,179,083 Bruche Nov. '7, 1939 2,189,321 Morton Feb. 6, 1940 

